The invention relates to the use of specific pyridine derivatives as catalysts for the dimerization of isocyanates (uretdione formation) and a process for preparing polyisocyanates having a high content of uretdione groups.
Aliphatic polyisocyanates which contain uretdione groups and are based on optionally branched, linear-aliphatic diisocyanates have a particularly low viscosity. Products based on aromatic, araliphatic or cycloaliphatic diisocyanates are generally highly viscous to solid substances which can be used, inter alia, as elimination-free, internally blocked crosslinkers in coating systems.
An overview of isocyanate oligomerization is given in J. Prakt. Chem./Chem. Ztg. 1994, 336, 185-200.
Tris(dialkylamino)phosphines (DE-A 3 030 513), if appropriate in combination with cocatalysts (DE-A 3 437 635), display a good selectivity for the formation of uretdione groups (uretdione selectivity).
DE-A 1 670 720 discloses the preparation of aliphatic polyisocyanates containing uretdione groups, in which tertiary phosphines having at least one aliphatic substituent and also boron trifluoride and its adducts are used as catalysts. It is indicated that high proportions of uretdione groups in the product can be obtained only at low conversions and reaction temperatures in the range from 50 to 80° C., with isocyanate trimers (isocyanurates and iminooxadiazinediones) and, especially at relatively high temperature, other by-products such as carbodiimides or uretonimines being formed at the same time. Uretonimines are particularly undesirable since they tend to liberate monomeric diisocyanate during storage.
DE A 10254878 describes the use of phosphines having at least one cycloaliphatic, P-bonded radical as catalysts for NCO dimerization. These catalysts display a significantly higher uretdione selectivity than other trialkylphosphines. The use of a specific case of these phosphines, viz. representatives bearing bicyclic radicals, for the same use is described in DE 10354544.
However, all phosphines suffer from the common disadvantage of air sensitivity, which makes their industrial use more difficult. Their uretdione selectivity is also strongly dependent on the reaction temperature and the degree of conversion of the monomer.
DE A 3 739 549 discloses catalytic NCO dimerization using 4-dialkylaminopyridines such as 4-dimethylaminopyridine (DMAP), but uretdione formation proceeds selectively only in the case of specific cycloaliphatic isocyanates such as isophorone diisocyanate (IPDI). Linear-aliphatic isocyanates such as hexamethylene diisocyanate (HDI) and also branched, linear-aliphatic isocyanates such as trimethylhexane diisocyanate (TMDI) and methylpentane diisocyanate (MPDI) give mainly strongly colored, heterogeneous reaction products when using DMAP and related compounds.
It is surprising that pyridine derivatives which are substituted by N atoms in the 3 and 4 positions on the pyridine ring, with the latter two N atoms being bridged by means of a two-membered, saturated (sp3-hybridized) carbon segment, are extremely effective catalysts for uretdione formation not only in the case of IPDI but also convert linear-aliphatic isocyanates such as hexamethylene diisocyanate (HDI) into virtually trimer-free polyisocyanates.